1. Field of Invention
This invention relates to methods and systems for correcting measurement depths in well log, particularly the LWD log.
2. Background Art
Subsurface or downhole logging may be accomplished after a well is drilled using a wireline tool or while drilling using a tool attached to a drill string. In wireline logging, a well tool, comprising a number of transmitting and detecting devices for measuring various parameters, is lowered into a borehole on the end of a cable or wireline. The cable, which is attached to some mobile processing center at the surface, is the means by which log data may be sent up to the surface. With this type of logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., based on the cable length while the tool is being pulled uphole.
Logging-while-drilling (LWD) collects data in a wellbore while the well is being drilled. By collecting and processing such information during the drilling process, the driller can modify or correct key steps in the operation, if necessary, to optimize performance. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are known as logging-while-drilling (LWD). Note that drilling operations may also use casings or coil tubings instead of conventional drill strings. Casing drilling and coil tubing drilling are well known in the art. In these situations, logging operations may be similarly performed as in conventional MWD or LWD. In this description, “logging-while-drilling” will be generally used to include the use of a drill string, a casing, or a coil tubing, and hence MWD and LWD are intended to include operations using casings or coil tubings. Furthermore, for clarity of illustration, in the following description, LWD will be used in a general sense to include both LWD and MWD.
In LWD logging, the measured data is typically recorded into tool memory as a function of time. At the surface, a second set of equipment records bit depth (based on drill string length or driller's depth) as function of time. When the data from the tools are made available uphole, the time-based measurements are converted to depth-based data by correlating the time information from the downhole tool with the time-depth information from the surface.
FIG. 1 shows a typical LWD system that includes a derrick 10 positioned over a borehole 11. A drilling tool assembly, which includes a drill string 12 and drill bit 15, is disposed in the borehole 11. The drill string 12 and bit 15 are turned by rotation of a Kelly 17 coupled to the upper end of the drill string 12. The Kelly 17 is rotated by engagement with a rotary table 16 or the like forming part of the rig 10. The Kelly 17 and drill string 12 are suspended by a hook 18 coupled to the Kelly 17 by a rotatable swivel 19. Drilling fluid (mud) 6 is stored in a pit 7 and is pumped through the center of the drill string 12 by a mud pump 9 to flow downwardly. After circulation through the bit 15, the drilling fluid circulates upwardly through an annular space between the borehole 11 and the outside of the drill string 12. Flow of the drilling mud 6 lubricates and cools the bit 15 and lifts drill cuttings made by the bit 15 to the surface for collection and disposal. As shown, a logging tool 14 is connected to the drill string 12. Signals measured by the logging tool 14 may be transmitted to the surface computer system 13 or stored in memory (not shown) onboard the tool 14. The logging tool 14 may include any number of conventional sources and/or sensors known in the art.
Note that while both wireline logging and LWD logging generally use similar methods to measure formation properties, their depth measurements are acquired differently. In wireline operations, the depth values come from direct measurements of the cable lengths, whereas with LWD logs, the depth-based data result from merging the time-based tool measurements and time-based driller's depth measurements. Driller's depth is based on the sum of the lengths of all pipe joints below the drillfloor plus the length of the bottom-hole assembly as measured while strapped at the surface.
FIG. 2 shows a schematic illustrating how a driller's depth is obtained on the surface. Briefly, the depth of the bit (or sensors) 23 in the well may be derived from the total pipe tally 21 minus the stick up length 22. However, the total pipe tally 21 may not correspond to the actual pipe length in the wellbore because the downhole environments (e.g., temperatures) are very different from those at the surface. Therefore, the driller's depth may not necessarily represent the actual depth of the LWD sensors downhole at all times.
Inaccurate LWD logging depths render it difficult to have reliable results from well-to-well correlations, correlations to offset well data, formation dip and formation thickness determinations. Incorrect depth measurements may also introduce artifacts and obstruct identification of geologic features. Therefore, there is a need in industry for a LWD depth measurement that is accurate, consistent between wells regardless of rig type or bottomhole assembly configuration, and independent of drilling mode.